Rheumatoid arthritis causes significant disability and mortality. The joint capsule or synovium undergoes profound phenotypic changes during this disease, resulting in the hyperplastic and invasive tissue known as rheumatoid pannus. Pannus leads to cartilage and bone destruction, and the synovial fibroblasts within pannus show a persistently activated state characterized by increased proliferation, decreased apoptosis, and sustained growth factor and matrix-degrading enzyme secretion. Precisely how these phenotypic changes are initiated and become perpetuated in synovial fibroblasts is not understood, and this lack of knowledge is an important problem because without this information, more selective approaches to the treatment of arthritis cannot be undertaken. Our long-range goal is to understand the pathogenesis of rheumatoid pannus. The objective of this application is to understand the mechanisms underlying the persistent activation of synovial fibroblasts. The central hypothesis is that the cytokine, macrophage migration inhibitory factor (MIF), induces sustained ERK-1/2 activation, invasive phenotype, and suppression of p53-dependent apoptosis in synovial fibroblasts. We have formulated this hypothesis on the basis of our discoveries showing that immunoneutralization of MIF prevents arthritis development in murine models, and that MIF induces a sustained pattern of activation of the ERK-l/2 MAP kinase cascade in the well-defined, NIH-3T3 cell line. MIF also has been shown to uniquely inhibit p53-dependent apoptosis. The rationale for this research is that once it is known how persistent pathways of cell activation become induced in synovial fibroblasts, new approaches for intervention in joint destruction may be devised. We will test our hypothesis and accomplish the objective of this application by pursuing the following two specific aims: 1) Define the Pathways by which MIF Activates Synovial Fibroblasts, and 2) Determine MIF's Role and Mechanism of Action in the Inhibition of p53-dependent Apoptosis. The proposed work is innovative because it capitalizes on two recently discovered and potentially unifying mechanisms, sustained ERK-1/2 activation and p53 functional inactivation, to explain the hyperplastic and invasive phenotype of rheumatoid pannus. It is our expectation that this work will define the molecular basis for the sustained pro-proliferative and invasive phenotype of rheumatoid synovial fibroblasts. These results will be significant because they will provide a precise understanding of the molecular pathways responsible for the erosive properties of synovial pannus and ultimately facilitate the development of new and selective strategies for the treatment of rheumatoid arthritis.